WO2005108004A1 - Device and method for the production of microstructures - Google Patents

Abstract

The invention relates to a device and a method for the production of microstructures, comprising a spindle driven in rotation about a spindle axis, on which a housing for clamping a workpiece is provided, and an actuator (30), which has a rapid drive (36), in particular a piezo-drive for generation of a rapid movement of a tool (32) in a direction essentially perpendicular to the workpiece surface, whereby the actuator (30) may be positioned by means of a further drive (24) for generation of a linear movement in a first direction along a workpiece surface for machining (24). The rapid drive (36) is coupled to the tool (32) by means of a guide device (38) which permits an adjustment of the tool (32) in the axial direction of the rapid drive (36) against a return force and which presents a high rigidity in a plane perpendicular thereto. The applied control technology permits a precise and reproducible production of microstructures using the dynamic system properties.

Description

 Device and method for producing microstructures

The invention relates to a device for producing microstructures, with a spindle which can be driven in rotation about its longitudinal axis and on which a receptacle is provided for clamping a workpiece, with an actuator which has a fast drive for generating a fast movement of a tool in a Workpiece surface has a substantially vertical direction, wherein the actuator can be positioned in a first direction along a workpiece surface to be machined by means of a further drive for generating a linear feed. The invention further relates to an actuator suitable for such a device.

Finally, the invention relates to a method for producing a microstructured surface on a workpiece rotatingly driven by a spindle using a tool driven by an actuator, which can be linearly positioned along the workpiece surface by means of a drive and perpendicularly in the direction of the workpiece surface by means of the actuator is movable.

A device and a method according to the aforementioned type are known from EP 0 439 425 B1.

The known device and the known method are used for producing contact lenses by a turning method. Then a blank for a contact lens to be machined is attached to a spindle. The workpiece, which is driven in rotation by the spindle, can be machined by means of a turning tool which is positioned by means of a piezoelectric drive. The lathe has a column-like rotary support, which is pivotally mounted about a pivot axis. A slide guide is attached to the rotating support. A tool slide is guided linearly on the slide guide and is movable in the radial direction to the swivel axis of the rotary support. A tool holder for a turning tool is preferably mounted on the tool slide in the form of a turning diamond. For rough adjustment of the turning tool, the tool slide can be moved along the slide guide with the aid of a motor drive with the aid of a motor drive. A piezoelectric is used for fine positioning of the turning tool Drive provided, which can consist of two piezotranslator devices. While a first piezotranslator device enables movement in the direction of the slide guide, a second piezotranslator device is designed for a movement perpendicular thereto. The actuating movements of the piezotranslator devices can be controlled as a function of the angle of rotation of the spindle about the spindle axis.

In this way, high-precision surface processing of lenses by means of a diamond turning tool is made possible, and surfaces which are not rotationally symmetrical can also be produced.

Similar devices for producing non-rotationally symmetrical surfaces by turning with the aid of a piezoelectric drive are known, for example, from US Pat. No. 5,467,675 or from GB 2,314,452 A.

With the known devices and the known methods, fine structure surface treatments can be carried out with the aid of diamond tools, but due to insufficient dynamic properties, these devices and methods are not suitable for precisely machining very hard, metallic materials. Such materials are required, for example, as mold materials for the production of lenses by hot pressing. These can be hard metal materials, for example. In such molds, for example, for the production of lenses for lighting purposes, which are used in so-called poly-ellipsoid headlights (PES headlights), very specific microstructures have to be introduced, which are transferred to the lenses in question during the shaping by hot pressing. This micro Structures act as micro-optical components in the lenses produced in order to maintain a certain predetermined light distribution characteristic in the headlamp.

The production of such shapes, which may consist of hard metal or gray cast iron, has so far only been possible in the intended shape using geometrically undetermined methods. For this purpose, the shapes in question are first machined in a macroscopic form by rotating the lenses in question. The mold surface is then polished if necessary. The areas of the shape which are provided for the production of scattering centers on the lenses to be produced therewith are subsequently produced, for example, by a blasting treatment with corundum in a plurality of work steps, with cover panels being used for those areas which are not to be microstructured in this way. After blasting, some surface treatment is carried out. Numerous manual work steps are therefore required to produce such a shape, which means very time-consuming and expensive processing in order to produce the desired surface structure. In addition, such a work sequence includes a multitude of error possibilities that adversely affect a reproducible setting of a special light distribution.

The previously known devices with piezo drives are not suitable for such a shaping because they do not have the necessary mechanical stability and the required dynamic properties for this in order to be able to work precisely when processing hard metallic materials with the required high cutting speeds. Although with the known devices could possibly. position-controlled work, but of course this is only possible with a sufficient distance from the first resonance frequency of the respective system, which is why working with the known systems is possible up to a maximum of about 1,000 Hz. However, the cutting speeds that can be achieved with this are not sufficient to achieve clean machining with sufficient surface quality in the case of the materials mentioned above, in particular if non-rotationally symmetrical surface structures of the type mentioned above are to be produced.

The invention is therefore based on the object of providing an apparatus and a method for producing microstructures, which also enables machining of hard metallic materials, such as hard metals and gray cast iron, at high cutting speeds and at the same time achievable non-rotationally symmetrical microstructures. Here, in particular, the production of molds for the production of lenses by hot pressing is to be made possible, which are suitable as lenses for PES headlights and are provided with micro-scattered lens structures.

This object is achieved according to the invention in a device according to the type mentioned at the outset in that the actuator is coupled to the tool via a guide device which allows the tool to be advanced in the axial direction of the fast drive against a restoring force and has a high rigidity in a plane perpendicular to it having. The object of the invention is completely achieved in this way.

By coupling such a fast drive with a specially designed guide device, which has a high degree of rigidity on all sides in a plane perpendicular to the direction of movement of the fast drive, an actuator-guided system for moving a turning tool can be created which has a sufficiently high resonance frequency combined with a high dynamic Has rigidity to enable machining of hard materials such as hard metals or gray cast iron with a sufficiently high cutting speed. This avoids vibrations that could otherwise easily arise when machining hard materials such as gray cast iron and that would impair the surface quality.

The fast drive is preferably a piezo drive.

In addition, other embodiments for the fast drive are also conceivable, such as a hydraulic drive. A fast drive in the sense of this application is understood to mean a drive which can carry out a fast controlled movement in the axial direction, but cannot carry out a controlled movement in a plane perpendicular thereto and can only absorb very small forces. In this context, “fast” is to be understood to mean that the drive, when actuated with a suitable control signal, can execute a movement with a frequency of at least 500 Hz or more. In a preferred development of the invention, the guide device has a static rigidity of at least 50 N / μm, preferably of at least 100 N / μm, in a plane perpendicular to the feed direction of the guide device.

Such a device can be used, in particular, to produce molds for hot pressing lenses for PES headlights, which have microlens structures on their surface or are “frosted”. This enables cutting speeds in the order of 60 meters when machining gray cast iron or hard metals per minute or more, which is enough to ensure clean machining of the non-rotationally symmetrical surfaces in an acceptable time using conventional insert inserts.

In an advantageous development of the invention, the guide device has a plunger which is movably held on first and second spring elements, the spring elements being largely unyielding in the radial direction of the plunger, but permitting deflection against the spring force of the spring elements perpendicular to the plunger axis.

In an advantageous development of this embodiment, the spring elements are designed as leaf springs, which are clamped on holders at their two longitudinal ends and are movable transversely thereto.

In an expedient development of this embodiment, the spring elements consist of feeler gauges which are connected to one another are clamped in the radial direction between the holder and the plunger.

As an alternative to this, a design as radially symmetrical spring elements, for example in the form of disc springs, is possible, the tappet being coupled in the center.

With such a construction, sufficient flexibility of the guide device in the direction of movement of the fast drive can be achieved with high rigidity in a plane perpendicular to it.

In an advantageous development of the invention, the tool is clamped at a first end of the plunger, while the plunger is biased against the fast drive at its second end opposite the tool.

This ensures the necessary restoring force against which the fast drive acts.

In an advantageous development of this embodiment, the tappet is biased against the fast drive by a fluid pressure.

Although the pretension could in principle also be achieved mechanically, for example by springs, this measure reduces the effect of the cutting force noise.

In an advantageous further development of this embodiment, the tappet is held in a housing, the tappet forming a pressure-tight closed space with the housing, which is connected to a Fluid pressure can be acted upon, the space being sealed off from the outside in the axial direction via a first membrane connected to the tappet on the piezo side and a second membrane connected to the tappet on the tool side, and wherein the first membrane has a larger effective area exposed to the fluid pressure than has the second membrane.

In an advantageous further development of this embodiment, the membranes consist of aluminum.

In this way, the restoring force for the piezo drive can be guaranteed in a simple and reliable manner and at the same time an effect of the cutting force noise can be reduced.

In an advantageous development of this embodiment, the housing has at least one damping element consisting of a sintered material, preferably a sintered metal. The damping element preferably has a portion of an open porosity.

In this case, a gap with a thickness of preferably 0.1 to 1 millimeter, which is filled with a damping means, can also be formed between the damping element and the facing membrane. This can be air, grease or oil.

These measures can further reduce the effects of cutting force noise. In an advantageous development of the invention, the piezo drive is clamped to a receptacle at its first end facing away from the plunger and is coupled to the plunger at its second end opposite the first end via a compensating element which compensates for alignment errors between the plunger axis and the longitudinal axis of the piezo drive.

Here, the second end of the piezo drive can rest against an associated centering of the plunger, for example via a convex element, in particular a ball or a spherical element.

In this way, the introduction of static and dynamic transverse forces in the radial direction, which can arise, for example, from alignment errors between the piezo drive and the plunger and from noise caused by cutting forces, is largely eliminated.

According to an alternative embodiment of the invention, the piezo drive is closed at its second end by a cover plate which is coupled to the plunger via a constriction.

In this way, the mass of the overall system can be reduced even further and, if necessary, a joining point can be saved, which is advantageous for the dynamics of the overall system.

According to a further embodiment of the invention, the drive allows a feed in a direction perpendicular to the spindle axis, while the actuator allows the tool to move in the direction of the spindle axis.

In this embodiment, the turning is carried out according to the type of face turning, while the tool is largely advanced in the direction of the spindle axis, that is to say in the z direction, by means of the piezo drive.

Such a structure is suitable, for example, for microstructuring molds for the production of lenses for PES headlights.

According to a further embodiment of the invention, the drive allows a feed in a direction parallel to the spindle axis, while the actuator allows the tool to move perpendicularly to it.

With such an arrangement, surface machining of the workpiece can be carried out by microstructuring on the outer surface in the longitudinal direction. Here, work is carried out according to the type of longitudinal turning, the actuator allowing the tool to be fed in a plane perpendicular to the spindle axis, for example in the x direction.

Such an arrangement is suitable, for example, for the microstructuring of tribologically stressed surfaces, such as bearing surfaces on the outside, in order to improve the grease absorption and thus to achieve significantly improved lubrication and emergency running properties. In an expedient development of the invention, an electronic controller for controlling the movement of the actuator is provided, which controls the movement of the actuator as a function of the angular position of the workpiece and the position of the actuator along the first direction relative to the workpiece.

In this way, non-rotationally symmetrical surface structures can be achieved with the aid of the piezo-controlled feed movement of the tool.

According to a further embodiment of the invention, the controller has a means for transforming a desired microstructure of the workpiece, which is to be generated in Cartesian coordinates, into a coordinate-transformed structure in polar coordinates, in which the manipulated values are stored as a function of polar coordinates, the rotation angle and the radius contain.

With such a coordinate transformation, the respective manipulated variable for the actuator can be determined in the event of the face turning.

The coordinate-transformed structure is preferably stored in a look-up table (LUT), from which the electronic control system derives an actuating signal which is fed to an amplifier for controlling the actuator.

In a preferred development of this embodiment, the electronic control has means for interpolating the amplifier supplied control signal as a function of position increments of the linear feed in the first direction.

While the inertia of the mechanical system in the working direction of the piezo drive makes interpolation superfluous and this automatically results in a certain smoothing, an interpolation of the control signals fed to the amplifier is sensible depending on the position increments of the linear feed. In this way, when imaging adjacent pixels of the microstructure, the formation of striations in the radial direction caused by the linear positioning movement in the radial direction is avoided.

In a further advantageous embodiment of the invention, the actuator has a first resonance frequency of at least 1,500 Hz, preferably of at least 2,000 Hz, particularly preferably of at least 3,000 Hz, the control device being designed to supply the actuator with a low-pass filtered or bandpass filtered signal. whose upper limit frequency is below the resonance frequency of the actuator.

With this embodiment, it is possible to work without a position control of the actuator at a high cutting speed, which is selected such that the cut-off frequency of the target microstructure to be generated is just below the resonance frequency of the actuator. In this way it is possible to work with an open system without position control at maximum cutting speed, at which a sufficient distance from the resonance frequency of the actuator is still maintained. The object of the invention is further achieved by an actuator for moving a tool on a lathe to produce a microstructured surface by means of a piezo drive, the piezo drive being coupled to the tool via a guide device which delivers the tool in the axial direction of the piezo drive against a restoring force allowed and has a high rigidity in a plane perpendicular to it.

With regard to the method, the object of the invention is achieved by a method for producing a microstructured surface on a workpiece rotated by a spindle using a tool driven by an actuator, which can be moved in the direction of the workpiece surface by means of the actuator and perpendicular to it along the Workpiece surface can be positioned linearly by means of another drive, with the following steps:

(a) providing a target microstructure for a workpiece to be machined,

(b) transforming the target microstructure into a file (look-up table, LUT) which, depending on the angle of rotation of the workpiece and the linear feed path of the tool along the workpiece surface, contains positioning positions in a direction perpendicular to the piezo-controlled positioning movement of the tool,

(c) Creating a spatial frequency analysis of the target microstructure and determining a maximum cutoff frequency of the signal for the position as a function of the angle of rotation, of linear feed of the actuator along the workpiece surface and the cutting speed,

(d) adjusting the cutting speed for turning the workpiece such that the maximum cut-off frequency of the target microstructure is below the first resonance frequency of the actuator,

(e) Driving the spindle and a drive for linear positioning of the actuator along the workpiece surface and microstructuring of the workpiece by delivering the tool against the workpiece surface by means of the actuator according to the manipulated values derived from the look-up table as a function of the cutting speed and the angle of rotation and the feed path of the actuator along the workpiece surface.

In this way, the method according to the invention ensures that the microstructuring of the workpiece can be carried out with the highest possible cutting speed and that the actuator is still operated below its resonance frequency. In this way, the system can be used up to the maximum cutting speed, which is predetermined by the resonance frequency of the actuator, without a position control being necessary for this.

In principle, the method according to the invention is also suitable for use with conventional devices for microstructuring workpieces by turning using actuator-controlled tools. Preferably, the invention However, the method is used in connection with a device according to the invention.

In an advantageous development of the method according to the invention, the signal for the delivery of the actuator is low-pass filtered or band-pass filtered if the cutting speed that can be set according to step (d) is not sufficient.

If the spatial frequency analysis of the target microstructure to be generated shows that very high frequencies are contained therein, such as occur, for example, in the case of sharp edge transitions, the cutting speed to be selected would normally be significantly reduced in order to maintain a sufficient distance from the resonance frequency of the system. In this case, it is ensured according to the measure mentioned above that even with such a target microstructure, work can be carried out at a high cutting speed. By filtering the signal beforehand, the target microstructure is smoothed, so to speak, to ensure that it is possible to work at a sufficiently high cutting speed, for example to be able to machine hard materials such as gray cast iron or hard metal.

According to a further embodiment of the invention, the target microstructure of the workpiece is generated by means of an algorithm in such a way that a low-pass-limited white noise results in the spatial frequency analysis.

Here, the target microstructure of the workpiece can be folded by means of a dot pattern generated by means of a random generator with a low-pass filter, preferably with a binomial filter.

With this configuration of the microstructure, it can already be ensured during the generation of the target microstructure that a low-pass filtered signal results which is particularly suitable for being able to work just below the resonance frequency of the actuator.

Alternatively, such a target microstructure can be generated by generating a randomly generated point pattern, which is transformed into a frequency space, is low-pass filtered and is then transformed back into the spatial space.

Such a target microstructure is particularly suitable for the production of a mold for producing a lens for a PES headlight by hot pressing, in which a scattering microlens structure is to be formed on the surface of the lens.

According to the method according to the invention, the actuator can advantageously be advanced in the direction of the spindle axis and positioned in a direction perpendicular to it by means of the first drive.

This enables processing according to the type of facing.

As an alternative to this, the actuator can also be positioned parallel to the spindle axis in the z direction by means of the first drive and the actuator is moved perpendicular to the workpiece surface.

With this design, a microstructuring of the workpiece according to the type of longitudinal turning is made possible.

As already mentioned above, the method according to the invention is particularly suitable for microstructuring an optical element, in particular a microlens structure or a diffractive structure, or for producing a mold for such an optical element.

In addition, the method according to the invention is suitable for numerous other microstructuring purposes on workpiece surfaces. This includes the microstructuring of a tribologically stressed surface, especially for a plain bearing.

It goes without saying that the features of the invention mentioned above and those yet to be explained below can be used not only in the combination specified in each case, but also in other combinations or on their own without departing from the scope of the invention.

Further features and advantages of the invention result from the following description of a preferred exemplary embodiment with reference to the drawing. Show it:

Figure 1 shows a device according to the invention in a highly simplified, schematic representation; Figure 2 shows a longitudinal section through an actuator according to the invention according to Figure 1;

FIG. 2a shows a detail of an embodiment of the actuator according to the invention modified in relation to FIG. 2 in the area of the connection between the actuator and the plunger;

FIG. 3 shows a perspective view of the actuator according to FIG. 2;

4 shows the frequency response and the phase response of the actuator with the tool according to FIGS. 2 and 3;

FIG. 5 shows an enlarged detail of a target microlens structure to be produced for the production of molds for producing a lens for a PES headlight;

FIG. 6 shows a reproduction of the target microstructure according to FIG. 5 as a pixel image;

FIG. 7 shows a spatial frequency analysis of the target microstructure according to FIG. 6 along the outer circumference with a radius of R = 34 mm;

FIG. 8 shows a basic circuit diagram with the control for the actuator;

Figure 9 is a schematic representation of the algorithm for controlling the actuator and Figure 10 shows a longitudinal section through an actuator according to the invention, which is slightly modified compared to the embodiment of Figure 2.

A device according to the invention is shown extremely schematically in FIG. 1 and is designated overall by the number 10.

The device 10 according to the invention is a lathe which is additionally equipped with an actuator 30 in order to enable a quick, piezo-controlled movement of a tool 32 with respect to a workpiece 16. The device 10 has a spindle 12 which can be driven to rotate about its spindle axis 25, as is illustrated by an arrow 28. A receptacle 14 in the form of a chuck for clamping a workpiece 16 is provided on the spindle 12.

On a machine bed 13 of the device 10, a drive 20 is provided along a guide extending in the z direction (parallel to the spindle axis 25), by means of which a tool slide 18 can be moved in the z direction, as indicated by the double arrow 26. A drive 22 is provided on the tool slide 18, by means of which a slide 24 can be moved in the vertical direction (y direction), as indicated by the double arrow 27. Finally, a further drive is provided on the slide 24, which enables a movement in the horizontal direction (x direction transverse to the spindle axis 25). Finally, the actuator 30 is accommodated on the slide 24, by means of which the tool 32 can additionally be moved in the z direction, as indicated by the double arrow 34.

A central control is provided for controlling the lathe and the actuator, which is indicated schematically by the number 17. The angular position of the spindle 12 can be monitored by means of a rotary encoder 31.

The device 10 can now be used for the microstructuring of the surface of a workpiece 16 in such a way that the workpiece 16 is driven to rotate about the spindle axis 25 by means of the spindle 12, while the actuator 30 is positioned in the x direction by means of the slide 24 and the tool 32 by means of the actuator 30 as a function of the angular position of the workpiece 16 detected by the rotary encoder 31 and of the location coordinate of the actuator 30 in the x-direction on the workpiece surface. In this way, a microstructuring of the workpiece surface can be achieved, wherein non-rotationally symmetrical surface structures can be produced with high precision.

With the construction described with reference to FIG. 1, workpieces can be machined in the manner of the face turning, i.e. the workpiece is driven in rotation and the tool 32 is predominantly transversely to the spindle axis in the horizontal direction (x direction) or radially to the workpiece 16 moves, while the rapid feed movement of the actuator 30 takes place in the z direction, that is, in the direction of the spindle axis 25. In addition, movements in other directions are of course possible, for example in the z direction for turning contours. It goes without saying that workpiece machining according to the type of longitudinal turning is also possible. For this purpose, the positioning movement for the actuator 30 takes place in the z direction by means of the drive 20, while the actuator 30 enables an infeed movement in a plane perpendicular thereto (x / y plane). For this purpose, the actuator is expediently positioned on the slide 24 in such a way that the piezo axis is aligned in the x direction (or possibly in the y direction). With such an arrangement, a microstructuring of a workpiece could thus be carried out on the outer surface (or possibly on the inner surface) in accordance with the type of external rotation or internal rotation.

The special structure of the actuator, which can be moved very quickly against a restoring force in the direction of the piezo axis, but which has a high degree of rigidity in a plane perpendicular to the piezo axis, is described in more detail below with reference to FIGS. 2 and 3.

The actuator shown as a whole in FIG. 2 and 3 and designated by the number 30 essentially consists of a piezo drive 36 which is accommodated on a housing and which acts on a tool 32 via a guide device 38. The piezo drive 36 only allows movement in the axial direction of the piezo drive, but cannot generate actuating forces in a plane perpendicular to it. The guide device 38 now ensures that the actuating movements of the piezo drive 36 in its axial direction can be transmitted directly to the tool 32, but that the entire actuator 30 has high rigidity in one plane perpendicular to the piezo axis. For this purpose, the guide device 38 has a plunger 40, the plunger axis 41 of which is aligned as precisely as possible with the longitudinal axis 37 of the piezo drive 36. The plunger 40 can be moved against the force of the first spring elements 44 and second spring elements 48 in the direction of its plunger axis 41 in the axial direction, but due to the special design and clamping of the spring elements 44, 48 it is practically unrelentingly suspended in a plane perpendicular to the plunger axis 41. The spring elements 44 and 48 are leaf springs made of wide spring steel of small thickness, as is known, for example, as a feeler gauge band. These leaf springs are clamped at their outer ends to holders, while the plunger 40 is fixed in the middle. The first spring elements 44 are clamped between ring-shaped holders 45, 46, while the second spring elements 48 are clamped between ring-shaped holders 49, 50.

Overall, this results in a possible axial deflection of the plunger in the order of magnitude of approximately 0.5 mm, while the plunger 40 is held perpendicular to it with a very high rigidity (static rigidity greater than 100 N / μm). In the case shown, the necessary restoring force on the piezo drive 36 is not generated mechanically but pneumatically. For this purpose, the plunger 40 is suspended within a housing 64 in such a way that a cavity 72 which is sealed off from the outside is reached and can be acted upon by compressed air via a compressed air connection 74. The cavity 72 is sealed in the axial direction by a first membrane 68 at its end facing the piezo drive 36 and by a second membrane 70 at the tool end. In this case, the membrane that acts on the outside (towards the ambient air) diameter in the first diaphragm 68, which faces the piezo drive 36, is significantly larger than the diaphragm diameter 70 on the opposite side. This results in a pressure difference through which the plunger 40 connected to the two membranes 68, 70 in the center is biased in the direction of the piezo drive 36. The prestressing force, which should naturally be greater than the acceleration forces generated by the piezo drive 36, can be set by the choice of the area ratios of the membranes 68, 70 and by the pressure applied.

The membranes 68, 70 preferably consist of relatively rigid aluminum elements and are in each case on their outer circumference against a thin counter membrane 69 or 71, which is pressed against the membrane 68 or 70 by the overpressure in the cavity 72. Furthermore, a sintered metal plate 66, which extends largely over the entire cross section of the cavity 72, is held at the axial end of the cavity 72, which faces the piezo drive, at a very short distance from the counter membrane 69. The sintered metal plate 66 consists approximately of sintered steel and has a certain proportion of open porosity.

In this way, the sintered metal plate 36 is permeable to air in order to be able to transmit the internal pressure prevailing in the cavity 72 to the membrane 68, but leads to a noticeable damping due to the thin channels which are formed by the open porosity, as a result of which the effects of the cutting force noise in Operation of the actuator can be significantly reduced. The tool 32 itself can be designed, for example, as a cutting plate, which is fastened to a tool holder 42, which is fastened (by means of a screw, not shown) to the outer axial end of the plunger 40, the fastening to the second spring elements 48 in the form of the crossed at the same time Leaf springs are made. The opposite end of the plunger 40 is firmly connected to the two first intersecting spring elements 44. For this purpose, a screw 80 is used, which extends through corresponding central recesses of the two crossing spring elements 44 with the interposition of a spacer 76 and an intermediate piece 78 into the end of the plunger 40 and is screwed thereto.

The piezo drive 36 is rigidly screwed at its end facing away from the plunger 40 via a piezo receptacle 62 to an end piece 60. At its end facing the plunger 40, the piezo drive 36 abuts the screw 80 via a ball 82 via a centering 86. In this way, any radial forces on the piezo drive 36 are avoided, which could be caused by an alignment error between the longitudinal axis 37 of the piezo drive 36 and the plunger axis 41. This ensures that the piezo drive 36 is only stressed in the direction of its longitudinal axis 37.

The unit formed from the piezo drive 36 and its suspension with end piece 60 is screwed directly through the holders 45, 46 through threaded bolts 52 with the interposition of spacer sleeves 54.

Overall, this results in a compact actuator 30, in which the piezo drive 36 counteracts a pneumatic reset Force is movable in the axial direction and has a high rigidity in a plane perpendicular to it.

Additional measures, such as a complete splash guard to protect the piezo drive 36 against cutting oil during turning, are not described here, since these are familiar to the person skilled in the art.

A modification of the connection between piezo drive 36 and plunger 40 can be seen in FIG. 2a. Here too, a compensating element 43 is provided between the piezo drive 36 and the plunger 40, which compensates for alignment errors between the longitudinal axis 37 of the piezo drive 36 and the plunger axis 41. Here, however, the compensating element 43 is formed as a constriction 47, which is coupled directly to an end plate 39 of the piezo drive 36 and to the plunger 40 and thus produces a direct, but laterally flexible connection between the piezo drive 36 and plunger 40.

A side support arm 84 can also be seen in FIG. 3, by means of which the actuator 30 can be fastened, for example on the slide 24.

FIG. 4 shows the measured frequency response and working range of the actuator according to FIGS. 2 and 3. In the present case, a piezo crystal of 40 μm maximum infeed was used as the piezo drive 36. The piezo drive 36 was excited with an amplitude of 1 V in a frequency range from 0 to 4000 Hz. The path as a function of the excitation voltage is shown in the upper diagram in FIG. 4. The vibrometer sensor picked up directly on the tool 32 Path signal shows a very constant amplitude response of approx. 2.5 μm / V from 0 to approx. 2250 Hz. The first resonance frequency is approx. 2400 Hz.

In the lower part of the diagram in FIG. 4, the phase response as a function of the frequency is shown. The result is a very linear phase response from 0 to 2250 Hz.

Because of its very good dynamic properties, the actuator according to the invention is therefore particularly suitable for the microstructuring of surfaces, in particular for the machining of hard materials which have to be processed at high cutting speeds. For example, with a structure amplitude of 5 μm, feed frequencies up to about 2.25 kHz can be realized. The maximum infeed frequency is somewhat lower at higher infeed amplitudes or very high material removal. A further improvement of the dynamics by using a conventional closed-loop control (instead of the open-loop control used) does not seem to be necessary and would not be possible to just below the resonance frequency due to the measured phase response.

The maximum travel of the actuator 30 used is approximately 40 μm, which means that the resonance frequency is approximately 2400 Hz. For many applications, however, a piezo drive with a maximum travel of 20 μm would also be sufficient. This would result in an increase in the lowest resonance frequency to approximately 3.5 kHz. The selection of correct processing conditions for the generation of a microlens structure will now be explained in more detail with reference to FIGS. 5 to 7.

The actuator 30, which was previously described in more detail with reference to FIGS. 2 to 4, was used in conjunction with a lathe of the index type from 1978, which had a high-precision spindle with a concentricity error of the order of magnitude of 0.4 μm.

The device was used to make a mold for hot pressing lenses for PES headlights. In order to maintain certain light distribution properties, the lens in question must be provided with microstructuring in the form of microlenses in selected surface areas. Such a microstructured or “frosted” lens achieves special light distribution properties when used in the poly-ellipsoid headlamp in question. The microstructured surface specified in the form is transferred to the lens surface by hot pressing during manufacture of the lens.

FIG. 5 shows an enlarged detail of such a microlens structure. The gray value of h _min with 0 and h _max with 255 is shown on the vertical axis, which corresponds to a structure depth of 0 to about 10 μm.

Such a structure can also be generated by a randomly generated point pattern folded with a binomial filter. To create such a structure, which is shown in Figure 6, for example, in an image of size 801 pixels by 801 pixels can be addressed with a random number generator. If no point was previously addressed at a location distance of n pixels (i.e. all gray values of the neighboring pixels in this area are 0), then the gray value at the addressed pixel is set to 1. The random generator runs through hundreds of thousands of times. This creates an image with randomly arranged dots of gray value 1, whereby two neighboring dots in each case do not fall below this limit distance. This image is then folded two-dimensionally with the binomial filter, possibly several times. Metaphorically speaking, the binomeal filter is "put on" at the center of each of the individual points with gray value 1. This creates an image with the randomly arranged "mountains" and "valleys", which very well simulates the shape of a microlens structure according to FIG. 5 The binocular filter of this type has low-pass properties, and the overall picture also has low-pass properties, which is very beneficial to the manufacture by the actuator 30 used.

In the case shown, the microstructure shown in FIG. 5 or FIG. 6 is now to be introduced into the surface of the mold by means of the actuator 30 using the facing process. It goes without saying that in this case a conversion of the target microstructure given in Cartesian coordinates into a polar coordinate system is necessary. This structure, which is implemented in polar coordinates, can be stored, for example, in the form of a look-up table (LUT). It specifies the manipulated variable G (c, n) for the actuator as a function of the angle of rotation (c) and the position (n) of the actuator in the radial direction. FIG. 7 shows a spatial frequency analysis that was created on a structure according to FIG. 6. Here, the spatial frequency on the outer circumference of the structure to be examined (in the case shown with a radius of 34 mm) is recorded by means of a Fourier analysis FFT (Fast Fourier Transformation). The surface spectrum shown over a circumferential section shows that the spatial frequency response is relatively constant from zero to about two periods per millimeter. From about 2.8 structures per millimeter, the spatial frequency response is close to zero.

The lower illustration in FIG. 7 now shows the conversion of the spatial frequency response recorded by the FFT analysis into an oscillation frequency response assuming a cutting speed of the tool of 50 m / min. This results in a linear drop in the spatial frequency response to near zero from approx. 2,350 Hz, while a very linear phase response is present up to approx. 1.7 kHz.

According to the invention, the cutting speed is selected such that the amplitude response has dropped to zero at the first resonance frequency of the actuator, which is approximately 2400 Hz according to FIG. 4. With the selected structure, a cutting speed of 50 meters per minute can be selected, since the spatial frequency response has dropped to almost zero at approx. 2,350 Hz. 6, which corresponds to the spatial structure according to FIG. 5, thus has the amplitude response of a low-pass-limited white noise and can advantageously be achieved with the actuator 30 according to the invention at a cutting speed of approximately 50 m / min. be generated. This also means that very hard metal materials, such as hard tallen possible that require a sufficiently high cutting speed, otherwise vibrations occur.

FIG. 8 shows the basic control of the device 10 for microstructuring during processing according to the type of face turning.

First, a stochastic microlens structure is created (see FIGS. 5 and 6). Alternatively, of course, other structures can be mapped, such as converted photographs, symbols, etc.

These structures are then transformed into polar coordinates.

The machining conditions are then selected and the machining is simulated using a spatial frequency analysis. The necessary control signals for the actuator and for the positioning of the actuator in the radial direction are then generated with a suitable cutting speed by a real-time computer for actuator control, which can be a PC with a signal processor card. For this purpose, the real-time computer evaluates the encoder pulses for the axis of rotation generated by the encoder 31 and the pulses generated by a further encoder for the radial axis and generates the start and end pulses for processing.

The spatial frequency analysis determines which cut-off frequency the generating target microstructure has. If the spatial frequency analysis reveals a very high cut-off frequency, this means that only a very low cutting speed is selected to ensure that the maximum cutoff frequency is below the first resonance frequency of the actuator. This would be the case, for example, if the target microstructure to be produced has sharp edges or the like.

In this case, it makes sense to first smooth the target microstructure to be generated by a low-pass filter or bandpass filter, for example by a Sobel filter, in order to obtain a low-pass filtered signal which can be processed in an advantageous manner with the actuator system according to the invention.

The target microstructure to be generated (cf. FIG. 6) should be square and be odd in the pixel representation, for example 801 by 801 pixels. From this square structure, a circle with a diameter of 801 pixels can be machined during turning. In order to obtain square pixels in the polar coordinates on the outer edge of the image, the image format should correspond approximately to the resolution of the encoder of the spindle axis divided by PI (the pixel accuracy). If a depth specification of 1 byte (= 255 gray levels) is selected, an encoder with 2,500 increments can be used, for example. The 255 gray levels should correspond to a depth of 0 to 10 μm later in the depth infeed of the actuator, for example.

The associated algorithm for such processing is now shown in FIG.

The target microstructure to be generated is converted into polar coordinates and with LUT as a polar coordinate image G (c, n) 2,500 x 401 pixels are saved, the actuating value for the actuator being specified with an accuracy of 1 byte. The manipulated value G can thus assume values between 0 and 255, which can correspond to a deflection of the actuator between 0 and a maximum of 40 μm or also a smaller range, for example from 0 to 10 μm.

The polar coordinate image according to FIG. 9 is stored in the LUT in BMP format.

According to FIG. 9, the angular position of the spindle 12 is recorded by means of the spindle C spindle and, based on an initial value “initiator spindle” after conversion into a digital value, recorded via the edge counter and processed as a “c value”.

The radial position of the actuator is also recorded via a rotary encoder Y and, after being converted into a digital value, processed using an edge counter.

The piezo drive is controlled by a real-time computer with the control values for the infeed in the z direction (path from h _min to h _max ). The amplifier receives its input values from the LUT and the values obtained from the encoders for the angular position and radial position via a digital / analog converter according to a suitable algorithm.

The amplifier input voltage U _min at g = 0 corresponds to h _min , while the amplifier input _voltage U _max at g = 255 corresponds to the maximum manipulated variable h _max . "Full radius" means the number of radial pulses when driving from R _max to R ₀ (i.e. from Outer radius to the middle). In FIG. 9, "floor (y)" means rounding down to the next even number.

With the example shown in Figure 9 the algorithm ^'amplifier via the D / A is controlled analog converter on the basis of the values from the LUT and from the values of the encoder for the spindle and C of the radial Y encoder for the radial position.

The algorithm uses an interpolation for two pixels lying directly next to each other in the radial direction in order to smooth the amplifier input voltage when the actuator is fed in by one pixel in the radial direction. As shown in the first box to the right of the polar coordinate image, the algorithm interpolates g = G (c, n + 1) * (1 - d) + G (c, n +2) * d. This then results in the digital value u for the amplifier input voltage according to u = U _min + g * (U _max - U _min ) / 255. This value is converted into an analog value for controlling the amplifier via the digital / analog converter.

There is therefore a linear interpolation of the manipulated values in the case of pixels lying next to one another in the radial direction, while no interpolation is used for the angle of rotation. In this way, otherwise recognizable "striations" in the direction of rotation are avoided.

No interpolation is required for the manipulated variable at a constant radial position, since the mechanical system of the actuator has a sufficiently high inertia to achieve smoothing in the circumferential direction. In FIG. 10, an embodiment of an actuator according to the invention which is slightly modified from the embodiment according to FIG. 2 is shown in longitudinal section and is designated overall by the number 30 '. Corresponding reference numbers are used for corresponding parts.

The structure of the actuator 30 'largely corresponds to the actuator 30 previously described with reference to FIG. 2. In contrast to the embodiment according to FIG. 2, the tool holder 42 has now been arranged such that the tool 32 is aligned exactly in the center. As a result, force components perpendicular to the axial direction, which occur in an eccentric arrangement according to FIG. 2, can be avoided. In this way, the cutting force noise is further reduced.

Furthermore, the centering between the plunger 30 ′ and the piezo drive 36 is formed with a spherical surface 88 on the screw 80 and a flat counter surface 90 on the piezo drive 36.

Finally, between the sintered plate 66 and the membrane 68 or the counter membrane 69, a narrow gap 73 with a width between approximately 0.1 and 1 millimeter, preferably between 0.1 and 0.5 millimeter, is formed, which is filled with a damping medium. This can be, for example, air, fat or oil.

This also further reduces the cutting force noise.

Claims

claims 1. Device for producing microstructures, with a spindle (12) which can be driven to rotate about its longitudinal axis (25) and on which a receptacle for clamping a workpiece (16) is provided, with an actuator (30) which has a fast drive (36 ) for generating a rapid movement of a tool (32) in a direction substantially perpendicular to the workpiece surface, the actuator (30) using a further drive (24) for generating a linear feed in a first direction (x) along a direction to be machined Workpiece surface can be positioned, characterized in that the fast drive (36) is coupled to the tool (32) via a guide device (38) which allows the tool (32) to be advanced in the axial direction of the fast drive (36) against a restoring force and has high rigidity in a plane perpendicular to it. 2. Device according to claim 1, characterized in that the fast drive is designed as a piezo drive. 3. Device according to claim 1 or 2, characterized in that the guide device (38) in a plane perpendicular to the feed direction of the guide device (38) has a static rigidity of at least 50 N / μm, preferably of at least 100 N / μm. 4. The device according to claim 1, 2 or 3, characterized in that the guide device (38) has a plunger (40) on the first and second spring elements (44, 48) is movably held, the spring elements (44, 48) being largely rigid in the radial direction of the plunger (40), but perpendicular to it in the direction of the plunger axis (41) a deflection against the spring force of the spring elements (44, 48) allow. Apparatus according to claim 4, characterized in that the spring elements (44, 48) are designed as leaf springs which are clamped at their two longitudinal ends on holders (45, 46, 49, 50) and are movable transversely thereto. Apparatus according to claim 5, characterized in that the spring elements (44, 48) consist of feeler gauges which are clamped together in the radial direction between the holder (45, 46, 49, 50) and the plunger (40). Device according to one of claims 1 to 4, characterized in that the spring elements are designed as radially symmetrical disks, in particular in the form of disc springs. Device according to one of claims 4 to 7, characterized in that the tool (32) is clamped at a first end of the plunger (40) and that the plunger (40) at its second end opposite the tool (32) against the fast Drive (36) is biased. 9. The device according to claim 8, characterized in that the plunger (40) is biased by a fluid pressure or spring pressure against the fast drive (36). 10. The device according to claim 9, characterized in that the ξJ tappet (40) is held in a housing (64), and that the plunger (40) with the housing (64) forms a pressure-tight closed space (72) which with a Fluid pressure can be applied, the space (72) being sealed off from the outside in the axial direction via a first membrane (68) connected to the tappet (40) on the piezo side and a second membrane (70) connected to the tappet (40) on the tool side wherein the first membrane (68) has a larger effective area exposed to fluid pressure than the second membrane (70). 11. The device according to claim 10, characterized in that at least one of the membranes (68, 70) consists of aluminum. 12. The device according to one of claims 2 to 11, characterized in that the piezo drive (36) is clamped at its receptacle (40) facing away from the first end on a receptacle (60), and with its second end opposite the first end via a Compensation element (43, 82), which compensates for alignment errors between the plunger axis (41) and the longitudinal axis (37) of the piezo drive (36), is coupled to the plunger (40). 13. The apparatus according to claim 12, characterized in that the second end of the piezo drive (36) via a convex Element, in particular a ball (82) or a spherical element, rests on the plunger (40). 14. The apparatus according to claim 12, characterized in that the piezo drive (36) is closed at its second end by an end plate (39) which is coupled to the plunger (40) via a constriction (47). 15. Device according to one of claims 10 to 14, characterized in that the housing (64) has at least one of a sintered material, preferably a sintered metal, existing damping element (66). 16. The apparatus according to claim 13, characterized in that the damping element (66) has a portion of an open porosity. 17. The apparatus of claim 15 or 16, characterized in that a gap (73) is formed between the damping element (66) and the facing membrane (69), which is filled with a damping agent. 18. The apparatus according to claim 17, characterized in that the gap is filled with air, fat or oil as a damping agent and preferably has a thickness of 0.1 to 1 millimeter. 19. Device according to one of the preceding claims, characterized in that the drive (24) feeds in a direction (x) perpendicular to the spindle axis (25). leaves and that the actuator (30) allows movement of the tool (32) in the direction (z) of the spindle axis (25). 20. Device according to one of claims 1 to 18, characterized in that the drive (20) allows a feed in a direction parallel to the spindle axis (25) (z), and that the actuator (30) a movement of the tool (32) allowed perpendicular to it (x, y). 21. Device according to one of the preceding claims, characterized in that an electronic control (17) is provided for controlling the movement of the actuator (30), which controls the movement of the actuator (30) as a function of the angular position (c) of the workpiece ( 16) and the position (s) of the actuator (30) along the first direction (x) relative to the workpiece (16). 22. The apparatus according to claim 21, characterized in that the controller (17) has a means for transforming a predetermined target microstructure to be generated of the workpiece into a coordinate-transformed structure in which the control values (G (c, n)) for the actuator (30) as a function of the angle of rotation (c) and the linear advance (n) in the first direction (x) along the workpiece surface. 23. The device according to claim 20 and 22, characterized in that the controller (17) provides a means for transforming a desired microstructure of the workpiece (16) to be generated in Cartesian coordinates into a coordinate-transformed structure in polar coordinates in which the manipulated values (G (c, n)) are stored as a function of polar coordinates (c, n), which contain the angle of rotation (c) and the radius (n). 24. The device according to claim 22 or 23, characterized in that the coordinate-transformed structure can be stored in a look-up table (LUT) from which the electronic control (17) derives an actuating signal (h) that an amplifier for control of the actuator (30) is supplied. 25. The device according to claim 24, characterized in that the electronic control (17) has means for interpolating the control signal supplied to the amplifier as a function of position increments of the linear feed (n) in the first direction (x). 26. Device according to one of the preceding claims, characterized in that the actuator (30) has a first resonance frequency (f _R ) of at least 1500 Hz, preferably of at least 2000 Hz, particularly preferably of at least 3000 Hz, and that the control device (17 ) is designed to supply the actuator (30) with a low-pass filtered or bandpass filtered signal, the upper limit frequency (f _G ) of which is below the resonance frequency (f _R ) of the actuator (30). 27. Actuator (30) for moving a tool on a lathe (10) for generating a microstructured surface by means of a fast drive, in particular a piezo drive (36), characterized in that the fast drive (36) is coupled to the tool (32) via a guide device (38) which allows the tool (32) to be advanced in the axial direction of the fast drive (36) against a restoring force and has a high degree of rigidity in a plane perpendicular to it , 28. A method for producing a microstructured surface on a workpiece (16) rotatably driven by a spindle (12) using a tool (32) driven by an actuator (30), which in direction (z) towards the workpiece surface by means of the actuator ( 30) is movable and can be positioned linearly along the workpiece surface by means of a further drive (24) and perpendicularly thereto, with the following steps: (a) providing a target microstructure for a workpiece (16) to be machined, (b) transforming the target microstructure into a file (look-up table, LUT), which is a function of the angle of rotation (c) of the workpiece (16 ) and from the linear advance path (n) of the tool along the workpiece surface contains positioning positions G (c, n) in a perpendicular working direction for actuator-controlled positioning movement of the tool (16), (c) creating a spatial frequency analysis of the target microstructure and determining one maximum limit frequency (f _G ) of the signal for the positioning position G (c, n) as a function of the angle of rotation (c), the linear feed (n) of the actuator along the workpiece surface and the cutting speed (v), (d) Adjusting the cutting speed (v) for turning the workpiece (16) such that the maximum cut-off frequency (f _G ) of the target microstructure is below the first resonance frequency (f _R ) of the actuator (30) (f _G <f _R ) (e) driving the spindle (12) and a drive (24) for linear positioning of the actuator (30) along the workpiece surface and microstructuring of the workpiece (16) by infeed of the tool (32) against the workpiece surface by means of the actuator (30 ) according to the manipulated values derived from the look-up table as a function of the cutting speed (v), the angle of rotation (c) and the feed path (n) of the actuator along the workpiece surface. 29. The method as claimed in claim 28, in which the signal for the delivery of the actuator is low-pass filtered or band-pass filtered if the cutting speed which can be set according to step (d) is not sufficient. 30. The method according to claim 28 or 29, in which the target microstructure of the workpiece (16) is generated by means of an algorithm such that a low-pass-limited white noise results in the spatial frequency analysis. 31. The method of claim 30, wherein the target microstructure of the workpiece (16) is generated by a randomly generated dot pattern, folded with a low-pass filter, in particular with a binomial filter. 32. The method according to any one of claims 28 to 31, in which the target microstructure is generated by a randomly generated dot pattern, which is transformed into a frequency space, is low-pass filtered and then transformed back into the local space. 33. The method according to any one of claims 28 to 32, in which the actuator (30) is advanced in the direction (z) of the spindle axis (25) and is positioned by means of the further drive (24) in a direction (x) perpendicular thereto. 34. The method according to any one of claims 28 to 32, in which the actuator (30) is positioned parallel to the spindle axis (25) in the z direction by means of the further drive and the actuator (30) perpendicular to it (x, y) against the workpiece surface is delivered. 35. A method for microstructuring an optical element, in particular a microlens structure or a diffractive structure, or for producing a mold for such an optical element, according to one of claims 28 to 34. 36. A method for microstructuring a tribologically stressed surface, in particular for a plain bearing, according to one of claims 28 to 34.